1. Field of the Invention
The present invention relates to a suspension controller, more particularly, to a suspension controller for altering suspension characteristic responsive to road surface conditions and vehicle running speed.
2. Description of the Related Art
When a vehicle is running at a low speed, generally, a suspension characteristic is to be maintained in a soft state to improve the ride comfort by decreasing the spring constant or damping force. On the other hand, when the vehicle is running at a high speed, the suspension characteristic is to be maintained in a hard state to improve control and stability by increasing the spring constant or damping force. Accordingly, various types of the suspension controllers for altering the suspension characteristic in response to the running speed of the vehicle have been proposed.
However, the aforementioned suspension controllers have brought such disadvantages as hunting in controlling the variable suspension characteristic because the vehicle speeds in both starting and terminating the suspension control are set to be the same. To cope with the above disadvantage, the vehicle speed value in starting the suspension control is set to be greater than that in terminating, or, in case the suspension control is executed at a single predetermined vehicle speed, a delay period is to be set in terminating the suspension control. For example, in Published Japan Patent Application No. sho 58-56907 entitled "Controller for Damping Force of Suspension", the suspension controller is provided with a vehicle speed detection means for detecting the actual vehicle speed, a high vehicle speed detection means for detecting whether the detected vehicle speed is a predetermined high vehicle speed to increase the damping force of the suspension to be above a predetermined high damping force, a low vehicle speed detection means for detecting whether the detected vehicle speed is a predetermined low vehicle speed for decreasing the damping force to be below a predetermined low damping force, and a maintenance means for maintaining the condition of the suspension to which the damping force is adjusted in response to the output from the high vehicle speed detection means or the low vehicle speed detection means. The suspension controller employs a timer for delaying the adjustment of the damping force of the suspension by a predetermined period when at least the decreasing vehicle speed is below a predetermined low vehicle speed.
However, the suspension controller according to the prior art possesses the following problems:
When a vehicle speed is increased to be not less than the predetermined high vehicle speed value, the suspension characteristic is altered to a hard state. In case the vehicle speed is decreased to be within the range which is below the predetermined high vehicle speed value and not less than the predetermined low vehicle speed value, the suspension characteristic is maintained in the hard state. With the above condition, if the vehicle passes over a single bump or dip, the shock being exerted on the vehicle body is not sufficiently absorbed, resulting in reduced ride comfort.
Since the conventional suspension control is executed only in response to the vehicle speed, if the vehicle passes over a single bump or dip with a speed in the vicinity of a border between the predetermined low vehicle speed and the high vehicle speed, the ride comfort is also reduced.